This invention relates to aqueous dispersions comprised of anionic water-soluble or water-swellable polymers, processes for making said dispersions, and methods of using said dispersions in water treating, dewatering, water clarification, papermaking, oil field, soil conditioning, food processing, mineral processing and biotechnological applications.
U.S. Pat. No. 3,658,772 discloses a process of copolymerizing a monomer composition of ethylenically unsaturated water-soluble monomers in an aqueous solution containing inorganic salts at a pH within the range from about 1 to 3.2 to produce a fluid suspension of dispersed solid-polymer particles. According to this patent, the polymers are comprised of at least 30 percent to about 95 percent of acrylic acid and from 0 percent up to 70 percent of acrylamide recurring units. The inorganic salt is present in an amount sufficient to precipitate the polymer as it is polymerized in solution. It is critical that the pH be within the stated range because the salt form of the polymer is more readily soluble. This patent is related to U.S. Pat. No. 3,493,500 which also discloses aqueous dispersions of acrylic acid/acrylamide copolymers in a continuous aqueous salt phase, where the critical pH is within the range of about 1 to about 4.
According to Japanese Patent Application Disclosure No. HEI 6-25540, an anionic dispersion in aqueous salt solution can be obtained by using dispersion polymerization to copolymerize no more than 15 mole % of a neutralized alkali (meth)acrylic acid, 2-acrylamide-2-methylpropanesulfonic acid, or the like and a polymerizable nonionic monomer such as (meth)acrylamide in an aqueous salt solution. An anionic dispersion can also be obtained by copolymerizing unneutralized (meth)acrylic acid or the like and (meth)acrylamide in an aqueous salt solution while agitating.
According to Japanese Patent Disclosure No. SHO 50-70489, there is a relationship between the sodium acrylate content of an acrylic acid/acrylamide copolymer and its solubility in an ammonium sulfate salt solution. Specifically, when the sodium acrylate content of the polymer is about 3 mole %, it is insoluble in a solution having a weight % ammonium sulfate concentration of greater than 22 percent. However, as the sodium acrylate content of the polymer becomes greater, higher and higher ammonium sulfate concentrations are required to precipitate the polymer. For instance, at a sodium acrylate content of about 16 mole % the ammonium sulfate concentration of the solution must be greater than 35 percent in order to precipitate the polymer.
There appears to be a recognition in the art that aqueous dispersions of anionic acrylamide copolymers may be formed at low pH without limitation on the anionic monomer content, but that the anionic monomer content is limited to about 15 percent at higher pH's where the anionic monomer is in the salt form. When the dispersed polymer contains greater than 15 mole percent anionic recurring units, the art recognizes that the pH of the aqueous dispersion must be low so as to keep the anionic units in their less soluble unneutralized form. The 15 percent limitation is recognized in Japanese Kokai Patent Number SHO 62(1987)-100548, SHO 62(1987)-20502, SHO 62(1987)-20511 and in EPO 183 466 B1. The art generally recognizes that the pH must be about 4 or lower, see U.S. Pat. Nos. 3,658,772 and 3,493,500 discussed above. International Publication No. WO 97/34933 mentions a pH of about 2 to about 5, but the highest pH stated in the examples is only 3.63 (Example 4). European Patent Applications EPO 604 109 A2 and EPO 630 909 A1 as well as U.S. Pat. No. 5,498,678 only exemplify cationic polymers.
Thus, there is a problem in that aqueous dispersions of anionic water-soluble or water-swellable polymers having an anionic content of greater than 15 percent are generally not available unless the pH of the dispersion is maintained below about 4. An anionic aqueous dispersion of a water-soluble or water-swellable polymer having a pH greater than 4 and an anionic content of greater than 15 mole percent would be desirable because these dispersions are generally utilized by admixing with water to disperse or dissolve the polymer, then utilizing the resulting diluted admixture in the desired application. It may be readily appreciated that the pH of the water may significantly affect the performance of these admixtures because of the pH sensitivity of the polymer. The problem is particularly acute when the anionic aqueous dispersion is admixed with water that is not highly alkaline e.g. neutral or slightly acidic water because the acidity of the dispersion itself may then render the resulting admixture even more acidic than the water. Therefore, it would be desirable to have an anionic aqueous dispersion with anionic content of greater than 15 mole percent that remains in the form of an aqueous dispersion at a pH greater than 4, preferably greater than 5, even more preferably greater than 6, so that the performance of the polymer could be materially increased.
Prior approaches to this problem have serious drawbacks. For instance, EPO 717 056 A2 discloses amphoteric copolymers of anionic cationic monomers in which it is preferred that the resulting polymer contain more cationic groups than anionic groups. Even if one skilled in the art were to proceed counter to this preference and prepare a polymer in which the anionic groups outnumbered the cationic groups, the inclusion of the cationic groups would still tend to dilute the anionic effect and to add extra cost. A similar dilution and cost disadvantage may result from copolymerization with a hydrophobic monomer such as in U.S. Pat. No. 5,605,970. Dilution and extra cost may also result when the anionic polymer is precipitated by a combination of kosmotropic salt and cationic organic salt as in copending U.S. Ser. Nos. 08/726,157; 08/723,628; and 08/726,158.
Disadvantages may also be apparent when a different sort of aqueous dispersion is prepared e.g. one in which the droplets of anionic polymer are not formed because of insolubility in a salt solution, but are instead the result of a phase separation process involving an incompatible second polymer. In these aqueous dispersions, salt is not necessary but instead the continuous phase contains a second polymer that is generally immiscible with the anionic polymer. For instance, in some cases the second polymer may be dilutive of the effect of the first polymer in a particular application, or may tend to viscosity the continuous phase to an undesirable extent. In this regard may be mentioned the following U.S. Pat. Nos. 4,380,600; 5,403,883; 5,480,934; 5,541,252; 4,778,836; 4,522,968; and 4,673,704. In this regard may also be mentioned the following European publications: EPO 573 793 A1; 624 617 A1; 169 674 B1; and 170 394 A2; as well as PCT document WO 95-11269.
In spite of the effort to make satisfactory anionic aqueous dispersions, the problem remains of producing anionic aqueous dispersions of high molecular weight water-soluble or water-swellable polymers that remain in the form of aqueous dispersions, i.e., the dispersed polymer remains insoluble at high charge and high pH and that have advantageously low bulk viscosities, high active polymer solids content, minimal quantities of dilutive material, and that dissolve or disperse readily regardless of the pH of the dilution water to give polymer admixtures which have the performance characteristics that are acceptable to the industry.